Surface Treatments of Metal Substrates

ABSTRACT

A method is provided for treating a metal substrate. The method comprises applying a metal layer to the substrate using a thermal spray process and electrochemically treating the metal layer to form a coating.

BACKGROUND

Devices such as mobile phones, tablets and portable (e.g. laptop or palm) computers are generally provided with a casing. This casing typically provides a number of functional features, for example protecting the device from damage.

Increasingly, consumers are also interested in the aesthetic properties of the casing. Furthermore, as devices such as mobile phones, tablets and portable computers are typically designed for hand held functionality, consumers also consider the weight of the device.

BRIEF DESCRIPTION OF DRAWINGS

By way of non-limiting examples, device casings and processes of manufacturing such casings according to the present disclosure will be described with reference to the following drawings in which

FIG. 1 is a flow diagram illustrating an example of a method of treating a metal substrate

FIG. 2 is a flow diagram illustrating another example of a method of treating a metal substrate

FIG. 3 is a sectional side view of an example of applying a metal layer on a metal substrate using wire arc spraying

FIG. 4 is a sectional side view of an example of a metal substrate following the thermal spray process of FIG. 1 or FIG. 2

FIG. 5 is a sectional side view of a metal substrate following the electrochemical treatment of FIG. 1 or FIG. 2

FIG. 6 is a partially cut away perspective view of a casing after the thermal spray process of FIG. 1 or FIG. 2

FIG. 7 is a perspective view of the casing of FIG. 6 after electrochemical treatment

DETAILED DESCRIPTION

The present disclosure describes a method of treating the surface of a metal substrate, for example the metal surface of a casing for a device. The method comprises forming a metal layer on the substrate using a thermal spray process followed by an electrochemical treatment to the metal layer to provide desired physical and aesthetic properties.

The use of thermal spray processes to apply a metal layer on the surface of the metal substrate can enhance the adhesion of the metal layer to the metal substrate when compared to application of metal oxides by thermal spray processes due to the relative smaller size of the metal to its oxide, therefore better filling of the pores of the metal substrate. This in turn can result in a more visually appealing product for highly porous metal substrates such as magnesium and its alloys, where multiple surface coatings can be required, resulting surfaces that are aesthetically undesirable, for example the coating looking cheap and “painted on”.

Furthermore, due to the substantially lower melting point of metals as compared to their oxides, energy requirements to perform the thermal spray process can be greatly reduced. The relatively lower melting point also makes thermal spray processes possible for highly reactive, light metals that can burn when exposed to high temperatures.

FIGS. 1 and 2 illustrate examples of methods of surface treating a metal substrate.

Referring to FIG. 1, a metal substrate is provided (110). The metal substrata may foe in the form of a casing for a device (180) as shown, for example, in FIG. 6. The casing (180) can foe formed using methods such as stamping, moulding, die-casting, thixo-molding or rolling into the desired shape of the finished product. In one example, the casing is formed of, for example, aluminium, magnesium, titanium, niobium, lithium, zinc or alloys thereof.

For example, the metal substrate (150) may be magnesium or its alloys. Use of magnesium in industry is typically limited due to a number of undesirable properties such as its high reactivity, tendency towards being corroded, high-temperature creep properties and flammability. Certain magnesium alloys can also provide further undesirable properties, for example magnesium alloys including iron, nickel, copper and/or cobalt increase tendency for the corrosion of the magnesium in the alloy.

However, magnesium is a strong, light weight and low density metal. These are particularly desirable properties for casings of electronic devices. Furthermore, although magnesium can be significantly more expensive than other light metals, casting and other formation processes are easier, more economical and faster with magnesium than for other light metals, for example aluminium.

Although, as noted above, certain magnesium alloys produce undesirable properties; the addition of small amounts of aluminium, zinc and/or manganese can positively alter the physical properties of magnesium. For example, the addition of manganese can increase corrosion resistance, while the addition of aluminium and zinc promote precipitation hardening, resulting in an alloy with a strength-to-weight ratio comparable to those of certain aluminium alloys and alloy steels. However, as discussed above, working and shaping the magnesium alloy is easier, more economical and faster than these alloys of comparable strength-to-weight ratio.

As seen in FIGS. 1 and 2, following the provision (110) of the metal substrate (150), the metal substrate (150) is then treated using a thermal spray process (120) to form a metal layer (160) on the substrate (150) such as that shown in the cross-section of FIG. 4.

Thermal spray processes (120) are processes in which melted or heated materials are sprayed onto a surface. The feedstock may be heated by electrical (plasma or arc) or chemical (combustion flame) means.

Thermal spray processes can provide thick layers of the feedstock over a large area at high deposition rates as compared no other processes such as electroplating, physical and chemical vapour deposition. In thermal spray processes, the feedstock materials are fed in powder or wire form, heated to a molten or send-molten state and accelerated towards the substrate in the form of particles, the particles typically sized from 3-200 nm.

A propelling fluid source such as compressed air for delivering a propelling fluid to an arc point, propelling molten metal particle created at the arc point of the two wires on to the substrate. The arc point is the location at which the wires come into contact an electrically arc based on their opposing electrical currents. The molten metal particle are created at the arc point which is then transmitted to the surface by the propelling gas. A directional nozzle may also be included to more accurately direct the molten metal onto the metal substrate.

One example of a thermal spray process is wire arc spraying, an example of which is shown in FIG. 3. In wire arc spraying, two consumable metal wires (200) are fed through respective wire guides (210). These wires (200, 205) are oppositely charged and at the point the wires come into contact, the arc point (220), producing an electrical arc based on their opposing electrical currents. The heat generated by this arc melts the incoming wires (200, 205), forming molten metal particles (230).

In order to transmit the molten metal particles (230) onto the substrate, a propellent is used, for example a gas stream (240) provided by a compressed gas source. A directional nozzle (250) may also be used to more accurately direct the molten metal particles (230) on to the metal substrate (150).

Other methods of thermal spray processes that may be used include plasma spraying, detonation spraying, flame spraying, high velocity oxy-fuel coating spraying (HVOF), warm spraying and cold spraying. The thickness of the metal layer formed by the thermal spray process may be in the order of 1-100 μm, and in particular 5-50 μm.

Feedstocks currently used for providing a metal finish to a substrate are typically metal oxides in order to provide certain physical and aesthetic properties to the substrate being treated. However, metal oxides have a significantly higher melting point when compared to their pure metals, for example aluminium has a melting point of around 660° C. whereas aluminium oxide (Al₂O₃) has a melting point of 2,072° C. As a result, significantly lower energy requirements are needed to produce molten metal required for thermal spray processes as compared to the energy requirements for metal oxides.

In addition, as a result of the lower temperatures of molten metals as compared to molten metal oxides, spray coating techniques using metals can be suitable for more reactive light metals such as magnesium which present a fire hazard and may burn when exposed to high temperatures.

Furthermore, the resulting metal layer applied by thermal spray processes better adheres to the porous metal substrate when compared to metal oxides applied by the same process due to the metal particles being smaller and therefore providing increased surface area of contact between the metal applied by the thermal spray process and the porous metal substrate.

As shown in FIGS. 1 and 2, following the application of a metal layer on the substrate by a thermal spray process (120), the metal layer (160) undergoes an electrochemical treatment to form a second layer or coating (170), as shown in FIGS. 5 and 8. Examples of possible electrochemical treatments include anodizing, micro-arc oxidation and electrophoretic deposition.

Properties of the electrochemically formed second layer or coating (170) such as porosity, hardness, colour, conductivity, wear resistance, corrosion resistance, thickness and adherence can be varied by varying the parameters of the electrochemical treatment. Such parameters include: the type of process used (e.g. anodizing, micro-arc oxidation or electrophoretic deposition); the chemical solution in which the treatment occurs (e.g. temperature and composition); the potential (e.g. pulse or continuous, direct current or alternating current, frequency, duration and voltage) and the processing time.

As shown in FIG. 2, following electrochemical treatment (130) the treated substrate may undergo a baking process (140) to set the layer formed by the electrochemical treatment (150). This baking process is typically undertaken in an oven at a temperature of nest less than 120° C., exceeding 180° C. and, more particularly, at about 170° C.

It will be appreciated that numerous variations and/or modifications may be made to the above-described embodiments, without departing from the broad general scope of the present disclosure. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive. 

1. A method of treating a metal substrate to form a coating, the method comprising applying a metal layer on the substrate, the metal layer applied by a thermal spray process, and electrochemically treating the metal layer.
 2. A method according to claim 1, wherein the metal substrate comprises aluminium, magnesium, titanium, niobium, lithium, zinc or alloys thereof.
 3. A method according to claim 1, wherein the thermal spray process comprises one of plasma spraying, detonation spraying, wire arc spraying, flame spraying or high velocity oxy-fuel coating spraying.
 4. A method according to claim 1, wherein after electrochemically treating the metal layer, baking the substrate.
 5. A method according to claim 1, wherein electrochemically treating the metal layer comprises anodizing, micro-arc oxidation or electrophoretic deposition.
 6. A method of applying a coating to a casing for a device, the method comprising heating a metal feedstock to form molten metal particles, accelerating the molten metal particles towards the casing to form a metal layer on the casing, and electrochemically treating the metal layer.
 7. A method according to claim 6 wherein the metal feedstock is in powder or wire form.
 8. A method according to claim 6 wherein heating the metal feedstock is achieved by electrical or chemical means.
 9. A method according to claim 6, wherein the metal casing is formed of aluminium, magnesium, titanium, niobium, lithium, sine or alloys thereof,
 10. A method according to claim 6, wherein the metal layer has a thickness of 5-50 μm.
 11. A method according to claim 6, wherein the molten metal particles have a diameter of 3-200 nm.
 12. A casing for a device having a coating, the casing comprising a metal substrata, a first metal layer on the metal substrate, the first metal layer applied by a thermal spray process, and a second layer termed by electrochemical treatment of the first metal layer.
 13. A casing according to claim 11, wherein he metal substrate comprises aluminium, magnesium, titanium, niobium, lithium, tine or alloys thereof.
 14. A casing according to claim 11, wherein the thermal spray process comprises one of plasma spraying, detonation spraying, wire arc spraying, flame spraying or high velocity oxy-fuel coating spraying.
 15. A casing according to claim 1, wherein electrochemically treating the metal layer comprises anodizing, electrophoretic deposition or micro-arc oxidation. 